Micellar catalysis is the acceleration of chemical reactions by the micelles of surfactants and mainly caused by the change in the concentrations of reacting components when they pass from a solution into micelles.1,2 Micellar catalysis is very much important because it provides environmentally friendly media.4-7 Chromic acid is traditionally used as oxidizing agents in organic synthesis, particularly in the oxidation of alcohol to carbonyl compounds.3 Different chelating agents such as picolinic acid (PA), 1,10-phenanthroline (phen), 2,2'-bipyridine (bipy) are used as promoter for chromic acid oxidation of different organic substrates in aqueous media. A variety of selective hexavalent chromium oxidizing agents such as pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), imidazolium dichromate (IDC) etc. have been developed for the oxidation of a wide range of compounds.8 All these reagents required some organic solvents like dichloromethane (CH2Cl2, DCM), acetone (CH3COCH3), dimethyl formamide (Me2NCHO, DMF). These are hazardous for skin contact, eye contact and for ingestion or inhalation.9 Solvents in particular make a large contribution to the atmospheric burden. So we have selected water as a solvent to avoid such hazards.
Materials and Reagents
Ethane 1,2-diol (98％, SRL, India), K2Cr2O7 (99％ AR, BDH), 1,10-phenanthroline (99.9％, Merck, India), sodium dodecyl sulfate (SDS) (AR, SRL, India), cetylpyridinium chloride (CPC) (98％, AR, SRL, India), TX-100 (98％, AR, SRL, India), H2SO4 (98％, Merck, India), HClO4 (65％, Merck, India) and all other chemicals used were of highest purity available commercially.
Procedure and Kinetic Measurements
Solutions of the oxidant and reaction mixtures containing known quantities of substrate (S) (ethane 1,2-diol), promoter 1,10-Phenanthroline (Phen) under the conditions [S]T ⨠ [Cr(VI)]T and [Phen]T ⨠ [Cr(VI)]T, acid and other necessary chemicals were separately thermostated (±0.1℃). The reaction was initiated by mixing requisite amounts of the oxidant with the reaction mixture. Progress of the reaction was monitored by the following the decay of the Cr(VI). The concentration of Cr(VI) at different time interval was measured by a titrimetric quenching technique using excess of standard Mohr’s solution and unreacted Fe(II) was estimated by a standard Ce(IV) solution using ferroin indicator.10 The pseudo-first order rate constant were calculated from the slopes of the plots of log[Cr(VI)]T versus time (t), which were linear at least for three halflives. The scan spectra and spectrum after completion of the reaction were recorded with a UV-VIS spectrophotometer [UV-2450 (SHIMADZU)]. Quartz cuvettes of path length of 1 cm were used. Under the experimental conditions, the possibility of decomposition of the surfactants by Cr(VI) was investigated and the rate of decomposition in this path was kinetically negligible.
Product Analysis and Stoichiometry
Under the experimental conditions, [ethane-1,2-diol]T ⨠ [Cr(VI)]T and [phen]T ⨠ [Cr(VI)]T (Subscript T stands for the total concentration) ethane-1,2-diol oxidised to hydroxyl ethanol confirmed by the melting point of hydrazone of hydroxy ethanal.11 [ethane-1,2-diol]T = 0.015 mol dm−3, [Cr(VI)]T = 0.001 mol dm−3 and [phen]T = 0.015 mol dm−3. In the experimental pH, H2CrO4 looses one proton and exist as HCrO4−. The overall stoichiometry of the reaction may be represented as:
RESULTS AND DISCUSSIONS
On the basis of [H+] and [ethane-1,2-diol] variation experiments the mechanism of unpromoted reaction is already established.12 The first step is the formation of ester followed by protonation and finally decomposition of protonated ester to yield product. This type mechanism is previously repoted.13-16
On the basis of [H+], [Phen] and [ethane-1,2-diol] variation experiments the mechanism of phen promoted reaction is given below. This type of phen promoted mechanism is previously repoted.
Scheme 1.Cr(VI) oxidation of ethane-1,2-diol in absence of phenanthroline.
Scheme 2.Cr(VI) oxidation of ethane-1,2-diol in presence of phenanthroline.
The colours of the final solutions in the absence of the promoter and in the presence of the promoter are different due to the presence of different types of Cr(III)-species. The colour of the final solution in the absence of the promoter under the experimental condition is pale blue (λmax = 412 nm and 592 nm) and the corresponding transitions17,18 are 592 nm for 4A2g (F)→4T2g (F) and 412 nm for 4A2g (F)→4T1g (F) of Cr(III) species. On the other hand, the colour of the final solution of the phen-catalysed reaction under the identical conditions is pale violet [λmax = 525 nm, for 4A2g(F)→4T2g (F) of Cr(III) species]. The spectra of the final solution of without promoter reaction and pure chromic sulphate solution in aqueous sulphuric acid media are identical. It proves that the final Cr(III)-species is simply Cr(III)-species for the without promoter reaction while for the phen-promoter reaction; the final Cr(III)-species is a different species, which is Cr(III)-phen complex. For phenpromoted reaction, there is a blue shift (Fig. 1) for the peak due to the transition 4A2g (F)→4T2g (F) compared to the final solution of the without promoter path. This blue shift is due to the presence of the strong field donor site, i.e. heteroaromatic N-donor site of phenanthroline. For the Cr(III)-phen complex, the peak due the transition 4A2g (F)→4T2g (F) merges with a charge transfer band (Fig. 1). For Cr(III)-aqueous species, the band at 270 nm due to 4A2g (F)→4T1g (P) transition appears as a shoulder on high energy charge transfer band.17,18 For Cr(III)-phen complex, the appearance of the charge transfer band at much lower energy for the proposed Cr(III)-phen complex is quite reasonable because of the favoured metal to ligand charge transfer. The vacant π* of the phen favours the metal to ligand charge transfer. The existence of the charge transfer band (metal to ligand) at lower energy for the phen-promoted reaction may be the cause of rate enhancement.
Fig. 1.(A) Absorption spectrum of unpromoted reaction mixture (after completion of reaction): [ethane-1,2-diol]T = 0.01 mol dm−3, [Cr(VI)]T = 5 × 10−4 mol dm−3, [H2SO4] = 0.5 mol dm−3. [phen]T = 0 mol dm−3. (The spectrum of the chromic sulfate is identical with this under the experimental condition.) (B) Absorption spectrum of the promoted reaction mixture (after completion of reaction): [ethane-1,2-diol]T = 0.01 mol dm−3, [Cr(VI)]T = 5 × 10−4 mol dm−3, [H2SO4] = 0.5 mol dm−3. [phen]T = 0.005 mol dm−3.
The scanned spectrum (Fig. 2) indicates the gradual disappearance of Cr(VI) species and appearance of Cr(III) species with an isobestic point at λ = 525 nm for the without promoter reaction λ = 515 nm for the with promoter reaction. Observations of this single isobestic point indicates the very low concentration of Cr(V) and Cr(VI) intermediates under the present experimental condition. In presence of promoter, Cr(VI)-phen is the active oxidant4,5,17 (Fig. 3).
Fig. 2.(A) Scanned absorption spectra of the reaction mixture at regular time intervals (12 min). Concentrations at the beginning of the reaction: [ethane-1,2-diol]T = 0.01 mol dm−3, [Cr(VI)]T = 5 × 10−4 mol dm−3, [H2SO4] = 0.5 mol dm−3. [phen]T = 0 mol dm−3. (B) Scanned absorption spectra of the reaction mixture at regular time intervals (3 min). Concentrations at the beginning of the reaction: [ethane-1,2-diol]T = 0.01 mol dm−3, [Cr(VI)]T = 5 × 10−4 mol dm−3, [H2SO4] = 0.5 mol dm−3. [phen]T = 0.005 mol dm−3.
Fig. 3.Absorption spectrum of reaction mixture with (B) and without phen (A) (in absence of substrate): [ethane 1,2-diol]T = 0 mol dm−3, [Cr(VI)]T = 5 × 10−4 mol dm−3, [H2SO4] = 0.5 mol dm−3. [phen]T = 0.005 mol dm−3.
Partition of ethane-1,2-diol is not equal to all types of surfactants (Fig. 4) due to different extent of hydrophobic portion of cationic (CPC), anionic (SDS) and neutral (TX- 100) surfactants. Hydrophobic portion is large in case of TX-100 than CPC. SDS has smallest portion of hydrophobic region. The partition of ethane-1,2-diol will be maximum in TX-100 and be minimum in SDS.
Fig. 4.Schematic representation of partitioning of substrate and active oxidant [AO+ = Cr(VI)-Phen complex] in (a) Cationic surfactant, (b) Anionic surfactant and (C) Neutral surfactant.
Active oxidant Cr(VI)-phen complex reacts with the substrate to form a ternary complex which experience a redox decompositions in a rate limiting step4-7,17,18 giving rise to organic product. Positively charged active oxidants are preferably accumulated in the anionic micellar phase of SDS due to electronic attraction. Considering the both facts, in presence of TX-100, the reaction simultaneously goes on in both in the micellar phase and aqueous phase and the rate is accelerated in the micellar phase because of the preferential accumulation of the reactants in the micellar phase. This type of observation is previously reported.19,20 In the presence of CPC, although the substrate is partitioned in the micellar phase, the approach of the active oxidants Cr(VI)-phen complex are repelled. Thus in the presence of CPC, the reaction is restricted to the aqueous phase which is depleted in the concentration of the substrate. This leads to the rate retardations compare to CPC. The catalytic effect of SDS is in between TX-100 and CPC.
Table 1.[Cr(VI)]T = 5×10−4 mol dm−3; [ethane-1,2-diol]T = 0.01 mol dm−3
Under pseudo-first order conditions, the monomeric species of Cr(VI) was found to be kinetically active in the absence of phenanthroline (phen) whereas in the phenpromoted path, the Cr(VI)-phen complex undergoes a nucleophilic attack by etane-1,2-diol to form a ternary complex which subsequently experience a redox decomposition leading to hydroxy ethanal and Cr(III)-phen complex. Combination of TX-100 and phenanthroline will be the ideal for chromic acid oxidation of ethane-1,2-diol in aqueous media.